Fuel cells are used to produce electricity when supplied with fuels containing hydrogen and an oxidant such as air. A typical fuel cell includes an ion conductive electrolyte layer sandwiched between an anode layer and a cathode layer. There are several different types of fuel cells known in the art; amongst these are solid oxide fuel cells (SOFC), polymer electrolyte membrane (PEM) fuel cells, and molten carbonate fuel cells. Fuel cells are regarded as highly efficient electrical power generators that produce high power density with fuel flexibility.
In a typical fuel cell, air is passed over the surface of the cathode layer and a fuel containing hydrogen is passed over the surface of the anode layer opposite that of the cathode layer. Oxygen ions from the air migrate from the cathode layer through the dense electrolyte to the anode layer in which it reacts with the hydrogen and CO in the fuel, forming water and CO2 and thereby creating an electrical potential between the anode layer and the cathode layer of about 1 volt. The fuel cells are typically stacked in series to provide higher voltages.
Each individual fuel cell may be mounted within a metal frame, referred to in the art as a retainer, to form a cell retainer frame assembly. The individual cell retainer frame assembly may then be joined to a metal separator plate, also known in the art as an interconnector plate, to form a fuel cell cassette. The cassettes may be stacked in series with a seal disposed between the sealing surfaces of each cassette to form a fuel cell stack.
Seals for fuel cell stacks require special properties such as a coefficient of thermal expansion comparable to those of the components of the SOFC stacks, a suitable viscosity to fill any gaps in the sealing surfaces of the cassettes, ability to maintain a hermetic seal at operating temperatures of 700° C. to 800° C., good chemical stability, and long term sustainability.
It is known that glass seals can provide sturdy bonded sealing joints between the fuel cell cassettes. However, when the fuel cell stack is cooled to room temperature (about 23° C.) from its typical operating temperature (about 700° C. to 800° C. for SOFC), residual stresses induced by a temperature gradient and/or a mismatch in the thermal coefficient of expansion (TCE) of different materials within the fuel cell cassettes may cause tensile stresses within the glass seals that may exceed the tensile strength of the joint, causing failure of the seals. Tensile stresses may also be formed by internal gas pressures within the fuel cell stack.
Since the glass seals are much stronger in compression than in tension, it has been determined that it is desirable to maintain a compressive force on the fuel cell stack (and thus on the glass seals) at all times, i.e. during operating and non-operating conditions. This may be accomplished with an end plate held in place with bolts that are torqued to provide a compressive force. However, due to the potentially large difference between the fuel cell stack operating temperature and room temperature, even a relatively small difference in the coefficient of thermal expansion between the bolts and the fuel cell stack may result in either an excessively high compressive force or no compressive force at all.
Previous approaches for providing compressive forces to fuel cell stacks using high temperature spring assemblies have been disclosed (U.S. Pat. No. 7,001,705 and US Pat Pub 2010/0233566). High temperature spring assemblies have proven to be an effective solution in fuel cell stacks with a smaller footprint (approximately 100 cm2 active area); however they may become impractical for fuel cell stacks with a larger footprint (approximately 400 cm2 active area). The spring assemblies need to exert considerably higher compressive forces to overcome tensile forces created by pressures within the large footprint fuel cell stacks during operation. The high temperature alloy components required may become prohibitively expensive. In addition, it may be much more difficult to apply the compressive force uniformly over the larger planer area which leads to increased complexity further increasing the cost of this type of loading mechanism.
Another approach using a bladder for providing compressive forces to a fuel cell stack has been disclosed (U.S. Pat. No. 6,258,475) which includes a fluid-filled pressurized bladder and a spring assembly. The spring assembly applies a first compressive force to a first load distribution plate while the bladder applies a second compressive force to a second load distribution plate. The flexible bladder and the fluid within the bladder must be configured to withstand the high temperatures generated by the fuel cell stack. These high temperature materials are generally quite expensive. In addition, a mechanism must be provided to pressurize the fluid within the bladder and to regulate the pressure within the bladder which leads to increased complexity further increasing the cost of this type of loading mechanism. Therefore, a simpler, lower cost device to apply a compressive force to a fuel cell stack is desired.